1. Field of the Invention
The present invention relates to an image forming apparatus that can be connected to a network, an image forming system including the image forming apparatus, and a control program thereof.
2. Description of the Related Art
In a conventional electro-photographic image forming apparatus such as a copying machine or a laser beam printer, the thickness of a sheet is so significant, in forming an image on the sheet, that the thickness of the sheet determines a quality of the image. This is especially the case in a conventional color copying machine because the color copying machine forms one single image by superposing toners of four colors onto one single sheet. As a result, the amount of toner formed on a sheet is significantly larger than in the case of a monochromatic copying machine. Accordingly, in a color copying machine, a difference in the thickness of a sheet considerably affects the quality of an image.
More specifically, in a general and common heating type fixing device in which toner is melted to be fixed, the greater the thickness of a sheet is, the amount of heat that is drawn and absorbed by the sheet during fixing becomes larger. Accordingly, the amount of heat that is used for melting the toner becomes smaller. Thus, with color image which uses a larger amount of toner than in the case of a monochromatic image, the toner many times is poorly fixed because the toner is not sufficiently melted. In order to prevent such poor fixing from occurring in a color copying machine, strict and accurate temperature control is especially necessary, compared to the case of a monochromatic copying machine.
In addition, it is also important, in performing a good and sufficient transfer of a toner image, to change transfer conditions (a transfer bias, for example) for transferring the toner image onto a sheet in accordance with the thickness of the sheet (namely, density of a material of the sheet).
In this regard, Japanese Patent Application Laid-Open No. 2001-192144 discloses a technology in which a sheet thickness determination sensor is provided to determine the thickness of a sheet upon turning on of power or mounting of a sheet feeding cassette, and thus the thickness of a sheet within the feeding cassette that is currently being mounted to the image forming apparatus is recognized. In addition, with this technology, a control apparatus controls a fixing temperature, a speed of image forming, a transfer bias, and the like, in accordance with the recognized sheet thickness.
In addition, the size of a sheet, as well as the thickness of a sheet, is a very significant factor in keeping a sufficient and good quality of an image. This is because just as in the case of the thickness of a sheet, the larger the size of a sheet is, the amount of heat drawn and absorbed by the sheet upon fixing becomes larger. Accordingly, the amount of heat used for melting the toner becomes smaller.
Further, in addition to the thickness and size of a sheet, the type and kind of a sheet is also a significant factor in keeping a good and sufficient quality of an image. In this regard, in the case of an OHP sheet, in which a transparency of a formed image is a very important factor, a fixing operation is performed at a slow fixing speed that is different from the fixing speed in the case of plain paper, in order to secure a sufficient transparency. In addition, in accordance with the recent wide-spread use of color image forming apparatuses, image forming is carried out at a slow fixing speed even in the case of a sheet other than an OHP sheet, such as a thick paper, in order to improve the quality of an image.
In this regard, for example, Japanese Patent Application Laid-Open No. 07-191510 (corresponding to U.S. Pat. No. 5,689,760) discloses a constitution in which a sensor for detecting the type of a sheet is provided, so that the fixing speed can be changed in accordance with a detection output from the sensor.
Further, there is a method in which information related to various kinds of sheets and control information are previously stored in an image forming apparatus so that when a user sets a sheet into a sheet cassette, information on the set sheet is selected.
Optimum image forming can be performed in relation to any type of sheet if the conditions for image forming are controlled on the basis of a plurality of factors including the thickness, the size, the type, and the like of the sheets used in an image forming apparatus in such a way. However, in this case, it is necessary to provide a sensor for detecting and determining each factor, in relation to each factor. Thus, the number of sensors becomes too large, resulting in complicating a control operation.
In addition, in the case where information related to various kinds of sheets and control information are previously stored in an image forming apparatus, the number of types of sheets to be stored becomes significantly large. Accordingly, a large-capacity memory needs to be provided in order to always store sheet information that a user may not use. Further, in the case of a sheet whose information is not previously stored, an optimum control operation cannot be performed.